I. Introduction to Time-of-flight Mass Spectrometers
Plasma source mass spectrometers analyze the contribution and identity of various components that make up a substance. Typically, this is done by dissolving the substance in a solution of known composition, and by vaporizing and ionizing the solution using hot plasma. The ionized plasma is then extracted as a continuous stream into a measurement chamber, such as a vacuum. Inside the measurement chamber, an electric deflection device affects the movement of the ions, and this movement is detected using an ion detector. In particular, the ion detector measures the flight time of ions traveling from the electrical deflection device, and provides a read-out indicating the mass/charge ratio of the solution's components. Mass spectrometers have proved extremely useful in applications such as chemistry, biology and environmental science, where they assist with the detection and identification of trace components of the substance being measured.
Generally, time-of-flight mass spectrometers are configured to receive the stream of the hot, ionized plasma, and periodically sample the stream by selectively and deliberately propelling packets of ions perpendicular to the stream. That is to say, time-of-flight mass spectrometers typically sample the ions by passing them through an ion pulser, which uses an electronic pulse to radically change the path of the sampled ions and propel them toward the detector. Since the ions may have different masses, and the same pulse is applied by the ion pulser to all of the sampled ions, the ion pulser causes the individual ions to have different velocities and arrive at the ion detector at different times. The time of arrival after sampling usually represents the mass/charge ratio, and the quantity of charge detected at a particular time represents a particular component's contribution to the measurement sample.
II. The Problem of Noise
Noise in mass spectrometers typically exists when the ion detector falsely detects stray particles. This can be caused, for example, by photons or neutral species generated by a plasma that impact the detector, as well as by ions that unintentionally escape from the ion pulser.
In the case of a time-of-flight mass spectrometer, the stream of continuously injected ions is normally channeled by the ion pulser between two charged plates, such that potential barriers created by the plates hold the ions between the two plates. When the ions are sampled, the ion pulser pulses one of the plates to have a much greater electric potential, which creates an electric field gradient that propels ions perpendicularly to the stream and toward the ion detector. Since photons neutral species have a neutral charge, they are not affected by the ion pulser and do not contribute significantly to noise. However, ions can inadvertently escape the potential barrier created by the charged plates at times when the ion pulser is not deliberately sampling the ion stream. Because of the charge of the plates and the charge of the escaped ions, the ions are inadvertently propelled toward the detector by the plates and may be correlated with the most recent pulse of the ion pulser and falsely determined to be a fast or slow ion that represents a particular mass. In other words, these stray "noise" ions are typically not distinguished from the deliberately sampled "signal" ions, and can arrive at the ion detector at near random times.
III. Plasma Gas Ions At The Ion Detector
Many plasma ion sources commonly ionize the solution being measured using argon gas plasma. The argon gas is ionized along with the solution, but to a much greater extent; argon ions are produced approximately one million times as frequently as each ion of the solution. Thus, plasma gas ions, and in particular, argon ions, usually form a very strong component of the signal ions which it is not desired to measure. The large quantity of argon ions, however, can also temporarily deplete charge carriers at the ion detector, which renders it difficult to detect some trace masses that form part of the solution. Thus, the plasma gas ions, due to their sheer numbers, contribute both to the overall background noise, and also to temporarily influence the ion detector.
There has existed a definite need for a mass spectrometer that reduces noise, and that therefore provides greater accuracy. What is needed is a mass spectrometer that has a mechanism for discriminating against stray charges that create noise, but that does not significantly affect the primary signal ions that are to be measured. Further, a need also exists for a system that filters specific ions which it is desired not to measure, such as argon, and reject them entirely. The present invention solves these needs and provides further, related advantages.